Shape Memory Effect in Cast Versus Deformation-ProcessedNiTiNb Alloys

Reginald F. Hamilton1 • Asheesh Lanba1 • Osman E. Ozbulut2 • Bernhard R. Tittmann1

Published online: 7 July 2015

� ASM International 2015

Abstract The shape memory effect (SME) response of a

deformation-processed NiTiNb shape memory alloy is

benchmarked against the response of a cast alloy. The

insoluble Nb element ternary addition is known to widen

the hysteresis with respect to the binary NiTi alloy. Cast

microstructures naturally consist of a cellular arrangement

of characteristic eutectic microconstituents surrounding

primary matrix regions. Deformation processing typically

aligns the microconstituents such that the microstructure

resembles discontinuous fiber-reinforced composites. Pro-

cessed alloys are typically characterized for heat-to-recover

applications and thus deformed at constant temperature and

subsequently heated for SME recovery, and the critical

stress levels are expected to facilitate plastic deformation

of the microconstituents. The current work employs ther-

mal cycling under constant bias stresses below those crit-

ical levels. This comparative study of cast versus

deformation-processed NiTiNb alloys contrasts the strain–

temperature responses in terms of forward DTF = Ms - Mf

and reverse DTR = Af - As temperature intervals, the

thermal hysteresis, and the recovery ratio. The results

underscore that the deformation-processed microstructure

inherently promotes irreversibility and differential forward

and reverse transformation pathways.

Keywords NiTiNb shape memory alloys (SMAs) �Thermal hysteresis � Deformation processing � Shapememory effect

Introduction

NiTiNb alloys are a class of NiTi-based shape memory

alloys (SMAs) distinguished by microconstituent mor-

phologies that facilitate a wide thermal hysteresis, more

than triple that of conventional NiTi SMAs. The wider

hysteresis is a result of stark increases in the reverse

transformation temperatures As and Af during shape

memory effect (SME) recovery after martensite deforma-

tion [1–13]. Those general observations were correlated to

the microstructure for heat-to-recover applications (mainly

couplings) in the inaugural works of Melton et al. [1, 2].

The influence of Nb addition to NiTi with respect to plastic

deformation giving rise to the wide hysteresis has been

expounded upon by Zhang et al. [3, 4], Zhao et al. [5–10],

and Piao et al. [11–13]. More recently, the NiTiNb classes

of SMAs have garnered interests due to the wide hysteresis

levels matching operating temperatures for civil engineer-

ing applications requiring pre-stressing or constraint

stressing [14–19]. Furthermore, the materials are expected

to exhibit damping potential [20, 21] as well as good

oxidation resistance [22].

The NiTiNb alloys are typically cast and subsequently

thermomechanically deformation-processed into useful

forms such as wires, rods, or sheets for practical applica-

tion [23]. As-cast microstructures generally consist of b–Nb ? eutectic NiTi (with dissolved Nb) in a cellular con-

figuration surrounding primary NiTi (with dissolved Nb)

matrix material, consistent with characteristic eutectic

microconstituent phases [3, 5, 9, 12, 18, 21, 24–31].

& Reginald F. Hamilton

rfhamilton@psu.edu

1 Department of Engineering Science and Mechanics,

The Pennsylvania State University, 212 Earth-Engineering

Sciences Building, University Park, PA 16802, USA

2 Department of Civil and Environmental Engineering,

University of Virginia, Charlottesville, VA 22901, USA

123

Shap. Mem. Superelasticity (2015) 1:117–123

DOI 10.1007/s40830-015-0024-1

Several investigations characterize the impact of defor-

mation processing conditions on the microstructure [19, 25,

31–35]. Ultimately, the findings show that the deformation-

processed microstructure is made up of oriented b–Nb-richmicroconstituents that are dispersed throughout the matrix

similar to aligned discontinuous fiber (or nanowire)-rein-

forced composites [36–38].

The functionality of deformation-processed NiTiNb

materials is primarily investigated via isothermal pre-

straining deformation of an initially martensitic or austenitic

microstructure and subsequent assessment of SME recovery

via heating [1, 4, 9, 13, 18, 26, 27, 39]. Transformation strain

recovery during heating is typically characterized without

load or under displacement constraint to assess the recovery

stresses. The b–Nb phase is presumably soft, and the critical

stresses during isothermal pre-straining deformation are

expected to plastically deform them, presuming that their

flow stress matches that for pure Nb which is estimated

between 150 and 200 MPa [1, 2, 23]. Plastic deformation of

microconstituents as martensite deforms necessitates an

increased thermal driving force for the reverse transforma-

tion that results in elevated As and Af temperatures and the

wide hysteresis [1, 2, 4, 13, 30], commonly referred to as a

stabilization effect [4, 10, 40–42].

The current work is an original investigation of the one-

way strain–temperature (e–T) response. Constant bias loadlevels are applied during thermal cycling, and the levels are

incrementally increased up to those reported to facilitate

plastic deformation of Nb-rich microconstituents. More-

over, this is the first comparative study of cast versus

deformation-processed NiTiNb alloys. Only the matrix

undergoes the martensitic transformation (MT) and hence

exhibits SME. In the cast alloy, large matrix regions exist

without the obvious appearance of microconstituent phases

within the regions. However, within the composite-like

deformation-processed microstructure, b–Nb fibers are

dispersed throughout the matrix and presumably can

interact differently with the MT morphology compared to

the cast microstructure. The aim of this comparative study

is to gain insights into the impact of b–Nb-rich phase using

the cast material e–T response as a benchmark for the

deformation-processed material response, which is the

prototypical NiTiNb SMA microstructure.

Materials and Methods

The compositions of both alloys are nearly equal to Ni47Ti44Nb9 at.%, which is the recommended ternary compo-

sition for wide hysteresis applications [23]. Atlantic Metals

and Alloys LLC supplied a cast alloy with the composition

Ni47.3Ti44.1Nb8.6 at.%. Medical Metals LLC supplied a

deformation-processed sheet with the composition

Ni47.7Ti43.5Nb8.8 at.%. The thermo-mechanical processing

methods for the strip are multiple thickness reductions

using cold work via rolling and annealing near the

recrystallization temperature (850 �C). Tensile specimens

with dog-bone geometry were electrical discharge

machined (EDM) from the cast materials. The gage

dimensions were length (l) = 10 mm, width (w) = 3 mm,

and thickness (t) = 1 mm. The thickness of the deforma-

tion-processed sheet material was t = 0.25 mm, and EDM

was utilized to micromachine dog-bone specimens with

l = 10 mm and w = 3 mm.

Specimens were mechanically polished for scanning

electron microscopy (SEM) and atomic force microscopy

(AFM) analysis. The materials were polished via SiC paper

with the grit size decreasing from 180 to 1200 and finally

polished using 0.02-lm colloidal silica. Microstructural

images were taken at room temperature using a Philips

XL30 ESEM scanning electron microscope. For higher

magnification imaging, a FEI NanoSEM 630 scanning

electron microscope was employed. SEM imaging was

performed in back-scattered electron mode. A Veeco

Metrology Autoprobe M5 atomic force microscope

(AFM) was used in contact mode and in air. The contact

force was maintained around 10–20 nN with an imaging

frequency of 1 Hz and a minimum detectable surface fea-

ture height of 1.2 A.

Load-biased thermal cycling experiments were con-

ducted using an MTS 810 servo-hydraulic load frame

equipped with a customized temperature cycling set-up.

Temperature gradients within the specimen were mini-

mized, and the heating and cooling rates were within

5–10 �C/min. The specimens were first heated to 150 �C,to ensure that the specimens were in the austenitic state.

The desired bias load was then applied and held constant.

The specimens were cooled to -90 �C and then heated to

150 �C. The external load for successive thermal cycles

was increased incrementally between 10 and 300 MPa. The

strain was calculated based on the displacement of the

actuator.

Results

Microstructure Characterization

Figures 1, 2 and 3 show the SEM micrographs of the cast

and deformation-processed microstructures. The cast mi-

crostructure in Fig. 1a, b exhibits the hypoeutectic char-

acter; the characteristic eutectic microconstituent is

arranged in a cellular configuration as boundaries encom-

passing regions of NiTi(Nb) matrix. The AFM image in

Fig. 1c reveals topography of the matrix and cellular

eutectic microconstituent. Locally, between the matrix and

118 Shap. Mem. Superelasticity (2015) 1:117–123

123

eutectic, well-defined boundaries exist and the cellular

regions are raised. The centers of NiTi(Nb) matrix regions

are the lowest height. Moving outward toward the eutectic,

the height rises approaching the eutectic-matrix boundary.

The height within the eutectic is relatively uniform. Fig-

ure 2a illustrates that the Nb-rich fibers are dispersed

throughout the matrix and oriented in the processing

directions, thus they appear as striations. The AFM images

of the deformation-processed material are shown in

Fig. 2b. The images reveal markedly refined topography

that is relatively smooth compared to Fig. 1c. Figure 2c

shows the transverse-section in which fibers appear as

speckles with spacing on the order of 100 nm. Figure 3a

exposes the characteristic eutectic lamellar and globular

mixture of Nb-rich b-phase and a-NiTiNb that is typical of

dissolved Nb [3, 5, 9, 12, 18, 21, 24–31]. The Nb-rich

fibers in the deformation-processed material are aligned

and discontinuous in Fig. 3b, yet the sizes remain consis-

tent with those in Fig. 3a.

Thermal Cycling With or Without Load

The thermal-induced martensitic transformation (TIMT) dur-

ing thermal cycling without load was evident for the cast

material in Fig. 4. The TIMT brings about exothermic and

endothermic events during cooling and heating respectively,

and thus, peaks arise in the heat flow versus temperature ther-

mograms measured using differential scanning calorimetry

(DSC) analysis. TIMT temperatures were Ms = -63.6 �C,Mf = -106.4 �C, As = -81.3 �C, and Af = 11.4 �C. For thedeformation-processed material, however, evidence for the

TIMT is not apparent in the DSC analysis.

The strain–temperature (e–T) responses in Fig. 5 show

the one-way shape memory effect behavior for cast and

Fig. 1 a SEM micrograph of the cast alloy cellular eutectic microconstituent arrangement, b SEM micrograph of the matrix encompassed by the

eutectic in the region within the box in a, c. 3D AFM image showing the varying surface topology

Fig. 2 a SEM micrograph of the deformation-processed microstructure with the Nb-rich fibers oriented in the rolling direction, b 3D AFM

image of the smooth surface and c SEM micrograph of the transverse-section of the composite fibers

Fig. 3 High-magnification

SEM micrographs of the

a eutectic microconstituent

phases in the cast microstructure

and b fibers with nano-scale

dimensions in the deformation-

processed microstructures

Shap. Mem. Superelasticity (2015) 1:117–123 119

123

deformation-processed alloys at increasing constant bias

stress levels. A e–T response for the 100 MPa bias load

level is evident for the cast material in Fig. 5a. A bias load

of 150 MPa was needed for the processed material in

Fig. 5b. Those bias stress levels were the minimum levels

that brought about measurable transformation strain. The

Ms temperatures for those bias stress levels for both

materials are equivalent and approximately equal to

-67 �C. For 100 and 150 MPa bias stress levels applied to

the cast materials, the slopes for the heating and cooling

segments of the e–T curves are nearly equal. At 300 MPa,

the slope for the heating segment differs from the cooling

segment in Fig. 5a. For the deformation-processed alloy

loaded at 150 MPa in Fig. 5b, the slopes for both curves

are equivalent. The slopes of the 300 MPa cooling and

heating e–T curves, however, exhibit differential slopes.

Moreover, each curve exhibits two slopes. An initial slope

appears vertical, and the stage is seemingly isothermal. A

second different slope follows in the cooling e–T curve.

The heating curve exhibits multiple slopes, albeit an

isothermal stage is indiscernible.

Metrics that characterize the e–T response are plotted

with increasing bias load in Fig. 6. Figure 6a captures the

effect of bias stress on the forward transformation

temperature interval DTF = Ms - Mf and the reverse

interval DTR = Af - As. For each material condition, the

DTF is less than DTR. The deformation-processed material

exhibits the narrowest DTF. The DTF for the cast material is

over 30 �C higher. The reverse transformation finish tem-

perature Af exhibits a marked increase (greater than 80 �C)when the bias load is increased from 150 to 300 MPa (see

Fig. 5). Consequently, for both materials, the DTR increa-

ses (by nearly 60 �C) from the lowest to highest bias load.

The dependencies of thermal hysteresis and recovery ratio

on bias stress level are illustrated in Fig. 6b. The thermal

hysteresis DTH is determined as the temperature differen-

tial at half the recovered strain during heating (see Fig. 5).

The hysteresis widens most when the stress is increased

from 150 to 300 MPa. The recovery ratio equals [(etr -eirr)/etr 9 100], where etr is the tensile strain accrued in the

cooling e–T curve and eirr is the unrecovered strain after

heating (see Fig. 5). The 150 MPa bias stress level facili-

tates a maximum recovery ratio for both materials and the

ratio drops for the 300 MPa level.

Discussion

The current findings demonstrate that despite vastly dif-

ferent microconstituent morphologies, plastic deformation

of the b–Nb-rich phase can have similar impacts on the

strain–temperature characteristic metrics for cast and

deformation-processed materials. In the NiTiNb class of

NiTi-based SMAs, the NiTi composition is expected to

dictate the transformation temperatures [1–4, 23]. Indeed,

the current results for similar Ni47.3Ti44.1Nb8.6 at.% (cast)

and Ni47.7Ti43.5Nb8.8 at.% (deformation-processed) com-

positions exhibit equivalent Ms temperatures, albeit only

the cast alloy exhibits the MT during stress-free thermal

cycling. A bias stress during thermal cycling was required

to bring about measurable shape memory behavior for the

deformation-processed material. The yield stress of the Nb-

Fig. 4 Normalized heat flow versus temperature thermograms from

differential scanning calorimetry analysis

Fig. 5 Strain–temperature

responses for thermal cycling

under constant stress for a the

cast alloy and b the

deformation-processed alloy.

The single and double arrows

depict the slopes during cooling

and heating, respectively. The

symbols are defined within the

text

120 Shap. Mem. Superelasticity (2015) 1:117–123

123

rich fibers has been estimated around 200 MPa [23], and

thus, impacts on the metrics become apparent when the

bias load is increased from 150 to 300 MPa. There is a

drastic increase in Af for both alloys. Consequently, the

reverse transformation temperature intervals DTR increase

starkly (by comparison, the forward transformation inter-

vals DTF are relatively consistent). Though the cast mate-

rial exhibits the largest hysteresis levels, the most marked

increase in hysteresis with bias load occurs for the defor-

mation-processed material. For both materials, a maximum

is apparent in the recovery ratios and the ratios drop with

that increase in hysteresis.

In as-cast materials, microstructure analysis shows that

the matrix regions are apparently free of the Nb-rich phase,

whereas the phase is dispersed throughout the matrix as

fibers in the deformation-processed materials. For the

deformation-processed composite-like microstructure, the

b-fibers exist as closely spaced reinforcements within the

matrix. Hence, an external bias load is required to facilitate

the one-way shape memory response and a measurable

strain–temperature (e–T) response. The composite

microstructure will increase the interfaces between

NiTi(Nb) and b-Nb phases [20] and which could act to

increase the dislocation density associated with the strength

mismatch between b-Nb fibers and matrix [35]. During

straining throughout the forward martensitic transformation

(MT), the Nb-rich fibers can have a strong coupling with the

NiTi(Nb) matrix [37, 38]. Coupled internal stresses can be

created between the fibers and the NiTi(Nb) matrix during

the forwardMT, and thus, an ‘‘internal-stress affected zone’’

can exist in the local vicinity of the fibers [36]. We envisage

that the NiTi(Nb) matrix regions within the cellular cast

microstructure can readily undergo the MT with the variant

morphology dictated by the external stress. The influence of

the eutectic microconstituent will likely be relegated to

inconsequential localized volume fractions adjacent to the

microconstituent boundaries. On the other hand, the ‘‘inter-

nal-stress affected’’ zones can dictate the MT variant

morphology due to the closely spaced fibers in the composite

arrangement of the deformation-processed material. Those

hypothesized contrasts are the focus of ongoing research

efforts. The contrasts are the basis for the following corre-

lations between the strain–temperature segments and ener-

getic contributions.

The slopes of the heating and cooling e–T curves for cast

and deformation-processed materials underscore the impact

of the differential microconstituent morphologies on the

energetics of the martensitic phase transformation. The e–T cooling and heating curves exhibit differential slopes with

increasing tensile bias load. The heating and cooling

e–T curves of the cast material exhibit a single slope, and thus

a single stage at each stress level. When the heating and

cooling e–T segments exhibit similar slopes at the lower bias

stresses, the responses are in accordance with crystallo-

graphic reversibility [43, 44]. At the highest stresses, the

heating and cooling curves for the cast or deformation-pro-

cessed alloy no longer exhibit equivalent slopes. The slope of

the cooling curve reflects continuous undercooling, which

overcomes elastic energy that otherwise resists the forward

MT [43]. The cooling curves exhibit the steepest slopes for

the deformation-processed material, and hence, elastic

energy storage is not predominant. During the initial stage in

the cooling curves for the process alloy, elastic energy

storage is compromised which apparently facilitates the

nearly isothermal growth of martensite [43].

The thermal hysteresis is directly related to energy that

is irreversibly dissipated during the MT [43–45]. Stored

elastic energy can be irreversibly dissipated during the

forward MT due to plastic deformation and thus the hys-

teresis widens, as the stored elastic energy is not available

to assist the reverse transformation [13, 43, 46]. The drastic

increase in Af, for both cast and deformation-process

alloys, reflects a martensite stabilization effect [4, 10,

40–42]. Martensite can be stabilized when it is pinned or

heavily dislocated such that the reverse transformation

requires a higher driving force [41, 42, 47]. It has been

Fig. 6 Characteristics

parameters a forward and

reverse transformation

temperature intervals and

b thermal hysteresis and

recovery ratio for different

levels of bias stress in Fig. 5.

The symbols are defined within

the text

Shap. Mem. Superelasticity (2015) 1:117–123 121

123

postulated that the b-phase can ‘‘lock’’ the martensitic

phase in NiTiNb alloys [48]. For the current results, the

reverse transformation temperatures must increase greatly

beyond the Af temperature for stress-free thermal cycling,

as well as the temperatures at the lowest bias stress levels.

The absences of a corresponding isothermal stage in the

heating e–T curves reveal differential transformation paths

for the forward versus reverse MTs for the deformation-

processed alloys. Differential transformation paths can

suggest non-thermoelastic MTs occur at the higher stress

levels [44, 49, 50]. The stark differential between forward

and reverse transformation temperatures intervals can fur-

ther point to a non-thermoelastic MT for both deformation-

processed NiTiNb alloys as well as cast alloys [43, 44, 49,

50]. The widening thermal hysteresis and diminished

recovery ratio at the highest bias stress imply marked

irreversibility that is common for non-thermoelastic MTs.

Conclusions

This comparative study of load-biased thermal cycling of

cast and deformation-processed NiTiNb alloys aimed to

correlate differential strain–temperature responses with

microstructure contrasts. The current findings support the

following conclusions.

• Close spacing between the aligned Nb-rich fiber

reinforcements in deformation-processed alloys brings

about a microstructure constraint that can suppress the

thermal-induced MT. A minimum biasing stress over-

comes the constraint.

• In the processedmicrostructure, the elastic energy storage

is relaxed. At the highest levels of constant stress, the

strain–temperature responseof the deformation-processed

alloys reflects that the forward and reverse MTs in

processed alloys take place in two stages. A single stage is

observed in the MTs in the cast alloys. The slopes of the

strain–temperature curves are steepest, and the initial

stage is seemingly isothermal for the processed alloy.

• The deformation-processed as well as the cast

microstructures facilitate a stabilization effect that

impacts the reverse transformation by increasing Af and

diminishes the recovery ratio. The slopes of the heating

and cooling segments are not equivalent. Plastic defor-

mation associatedwithNb-rich fibers can readily occur at

the 300 MPa bias stress level, considering that it exceeds

the reported flow stress of Nb. Hence, the transformation

may become non-thermoelastic.

Acknowledgments This study has been supported by the Mid-

Atlantic Universities Transportation Center (MAUTC) Pooled

Research Program issued by the Research and Innovative Technology

Administration of the US DOT (Grant No. DTRT12-G-UTC03). The

authors would like to thank Marius Schraff (Eidgenossische Tech-

nische Hochschule, Zurich, Switzerland) and Xiaoning Xi (Penn

State) for their help in acquiring AFM images.

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